In the early 1990's, one thing was fairly certain about the expansion of
the Universe. It might have enough energy density to stop its expansion and
recollapse, it might have so little energy density that it would never stop
expanding, but gravity was certain to slow the expansion as time went on.
Granted, the slowing had not been observed, but, theoretically, the
Universe had to slow. The Universe is full of matter and the attractive
force of gravity pulls all matter together. Then came 1998 and the Hubble
Space Telescope (HST) observations of very distant supernovae that showed
that, a long time ago, the Universe was actually expanding more slowly than
it is today. So the expansion of the Universe has not been slowing due to
gravity, as everyone thought, it has been accelerating. No one expected
this, no one knew how to explain it. But something was causing it.

Eventually theorists came up with three sorts of explanations. Maybe it was
a result of a long-discarded version of Einstein's theory of gravity, one
that contained what was called a "cosmological constant." Maybe there was
some strange kind of energy-fluid that filled